It is extremely difficult to stably disperse a carbon black powder in water at a high concentration due to hydrophobicity and low wettability with water. This is because the surface of carbon black has an extremely small amount of functional groups which have high affinity to water molecules (e.g., hydrophilic oxygen-containing functional groups such as a carboxyl group and a hydroxyl group). Various attempts have been made to improve the water-dispersibility of a carbon black powder by subjecting the carbon black powder to oxidization to produce hydrophilic functional groups on the surface of the carbon black powder.
An inkjet ink pigment has been produced by hydrophilizing the surface of a carbon black powder by various methods (for example, see Patent Document 1 (JP-A-2006-328137)).
In the method disclosed in Patent Document 1, a carbon black powder aqueous dispersion is produced by treating the surface of a carbon black powder with a chemical modifier (e.g., sodium peroxodisulfate) in an aqueous medium, neutralizing the acidic groups that are produced on the surface of the carbon black powder, and subjecting the resulting slurry to disaggregation. The method disclosed in Patent Document 1 provides a carbon black powder aqueous dispersion in which the amount of precipitates is reduced by forcibly disaggregating coarse particles (particles having a large particle size) dispersed in the solvent.